Method for forming a carbide layer of a V-a group element of the periodic table on the surface of an iron, ferrous alloy or cemented carbide article

ABSTRACT

A method for forming a carbide layer of a V-a group element of the periodic table on the surface of an iron, ferrous alloy or cemented carbide article in a treating molten bath, comprising heating a boric acid or borate up to its molten state to form a molten bath, dipping a metal containing a V-a group element into the molten bath, applying an electric current to the molten bath through said metal, which works as its anode, for anodically dissolving said metal into the molten bath to prepare the treating molten bath, and immersing the article in the treating molten bath, thereby forming a very hard carbide layer of said Va group element on the surface of said article. The method of this invention can form a very smooth carbide layer on the surface of the article.

Komatsu et al METHOD FOR FORMING A CARBIDE LAYER OF A V-A GROUP ELEMENT OF THE PERIODIC TABLE ON THE SURFACE OF AN IRON, FERROUS ALLOY OR CEMENTED CARBIDE ARTICLE Inventors: Noboru Komatsu, Toyoake; Tohru Arai, Nagoya; Yoshihiko Sugimoto, Nagoya; Masayoshi Mizutani, Nagoya, all of Japan Kabushiki Kaisha Toyota Chuo Kenkyusho, Nagoya, Japan Filed: Apr. 27, 1973 Appl No.: 355,278

Assignee:

Foreign Application Priority Data May 4,1972 Japan 47-43730 Apr. 12,1973 Japan 48-40822 References Cited UNITED STATES PATENTS 8 1960 Steinberg et al 204/39 1 Dec. 30, 1975 3,232,853 2/1966 Cook l48/6.1l 3,260,659 7/1966 Willing et al. 204/146 3,514,272 5/1970 Cook 204/39 3,620,816 11/1971 Rausch et a1. 117/119 3,719,518 3/1973 Komatsu et al 117/113 3,795,537 3/1974 Van Thyne et a1..... 148/6.11 3,814,673 6/1974 COOk 204/39 Primary ExaminerDouglas J. Drummond Assistant ExaminerMichael W. Ball Attorney, Agent, or FirmWenderoth, Lind & Ponack A method for forming a carbide layer of a V-a group element of the periodic table on the surface of an iron, ferrous alloy or cemented carbide article in a treating molten bath, comprising heating a boric acid or borate up to its molten state to form a molten bath, dipping a metal containing a V-a group element into the molten bath, applying an electric current to the molten bath through said metal, which works as its anode, for anodically dissolving said metal into the molten bath to prepare the treating molten bath, and immersing the article in the treating molten bath, thereby forming a ABSTRACT very hard carbide layer of said V-a group element on the surface of said article. The method of this invention can form a very smooth carbide layer on the surface of the article.

13 Claims, 3 Drawing Figures US. Patent Dec. 30, 1975 Sheet 1 of2 3,930,060

IMM and METI-IOD FOR FORMING A CARBIDE LAYER OF A V-A GROUP ELEMENT OF THE PERIODIC TABLE ON THE SURFACE OF AN IRON, FERROUS ALLOY OR CEMENTED CARBIDE ARTICLE This invention relates to a method for forming a carbide layer of a V-a group element of the periodic table on the surface of an iron, ferrous alloy or cemented carbide article, and more particularly it relates to the formation of the carbide layer on the surface of the article immersed in a treating molten bath. The

nese Pat. application Ser. No. 44-87805). The method can form a uniform carbide layer and is highly productive and cheap. The carbide of a V-a group element, such as vanadium carbide (VC), niobium carbide (Nb) and tantalum carbide (TaC) has a very high hardness ranging from Hv 2,000 to I-Iv 3,000. Therefore, the carbide layer formed, represents a high value of hardness and a superior .resistance performance against wear and is thus highly suitable for the surface treatment of moulds such-as dies and punches, tools such as pinchers and screwdrivers, parts for many kinds of tooling machines and automobiles parts subjected to wear. I

Further, the carbide of a V-a group element is much harder and less reactive with iron or steel at a high temperature than the tungsten carbide forming cemented carbide.

Therefore, the formation of the carbide layer of a V-a group element on the surface of a cutting tool composed of cemented carbide greatly increases the durability of the tool.

The method mentioned above, however, has a drawback. The method uses a treating molten bath containing metal particles. Said metal particles need a relatively long time to dissolve into the bath, and undissolved metal particles happen to deposit into the carbide layer formed and make the surface of the article treated rough.

Therefore, it is the principal object of this invention to provide an improved method for forming a carbide layer of a vV-a group element on the surface of an iron, ferrous alloy or cemented carbide article in a treating molten bath.

It is another object, of this invention to provide a method for forming a metallic carbide layer with denseness and uniformity and without any undissolved treating metal particles on the surface of the article.

. It is-still anotherobject of this invention to provide a method for forming a carbide layer, which is safe and simple in practice and less expensive.

It is a still further object of this invention to provide a treating molten bath which is capable of forming a carbide layer .having a smooth surface on the iron, ferrous alloy or cemented carbide article.

Other objects of this invention will appear hereinafter.

The novel features that are considered characteristic of the invention are set forth with particularity in the appended claims. The invention, itself, as to its method of operation, together with additional objects and advantages therefore, will best be understood from the following description of specific embodiments when read in connection with the accompanying drawings, in which:

FIG. 1 is a photomicrograph showing a vanadium carbide layer formed on the surface of a carbon tool steel according to Example 1;

FIG. 2 is a photomicrograph showing a niobium car bide layer formed on the surface of a carbon tool steel according to Example 2;

FIG. 3 is an X-ray diffraction chart of the layer formed on the surface of a cemented carbide according to Example 3.

Broadly, the present invention is directed to an improvement of the method for forming a carbide layer of a V-a group element of the periodic table on the surface of an iron, ferrous alloy or cemented carbide article in a treating molten bath and is characterized in that v the treating molten bath is prepared by anodically dissolving a Va group element into the molten boric acid or borate. Namely, the method of the present invention comprises heating a boric acid or borate up to its molten state to form a molten bath, dipping a metal containing a V-a group element into the molten bath, anodically dissolving said metal into the molten bath to prepare the treating molten bath and immersing the article in the treating molten bath for forming a very hard carbide layer of said V-a group element on the surface of said article.

Upon intensive investigation of the mechanism for forming a layer on the surface of an article by diffusing a V-a group element from the treating molten bath composed of a boric acid or borate and a metal powder containing the V-a group element, it was found the main source of the V-a group element forming the layer came from the dissolved element in the treating molten bath rather than directly from the solid metal particle undissolved. Namely, the V-a group element contained within the metal powder is dissolved into the treating molten bath, and then said dissolved V-a group element reaches to the surface of the article and diffuses into the article for forming the carbide layer with the carbon contained within the article. It is not necessary for the treating molten bath to contain the metal powder, and it is enough for the bath to contain a V-a group element dissolved therein. I

To dissolve the V-a group element into the treating molten bath for forming the carbide layer having a smooth surface, it is considered to clip a block of metal without contacting with the article to be treated instead of the metal powder mentioned above. However, the use of a block of metal instead of the metal powder greatly reduces the whole surface area of the metal so that the velocity of the dissolution of the V-a group element decreases remarkably. The metal block deteriorates by reacting with oxygen in the air before dissolving enough V-a group element into the treating molten bath. By experiments, we found that said deteriorates of the metal occurs on the metal block having a diameter of 2mm or larger than 2mm. In order to prevent deterioration of the metal, it is considered to use a thin film of the metal instead of the metal block or to employ an inert gas atmosphere for covering the treating molten bath and preventing oxygen from being 3 absorbed in the treating molten bath. However, in the former case, said thin film of the metal is not easily obtained and the later requires complicated and costly equipment and takes a long time to dissolve enough V-a group element for the treatment.

We have overcome the shortcomings mentioned above by anodically dissolving a V-a group element from a large block of the metal containing the V-a group element. Namely, using said large metal block as an anode in the molten bath, said boron oxide is electrolyzed by using the vessel of the bath as its cathode. By the anodic dissolution of a V-a group element, the treating molten bath of the present invention is prepared. The carbide layer of a V-a group element can be formed by immersing an article in the treating molten bath. The formation of the carbide layer on the surface of an article may be carried out during the anodic dissolution of a V-a group element into the treating molten bath.

The bath, consisting of the substances of, boric acid (B or borate, sodium borate (borax) (Na B O potassium borate (K B O and the like and the mixture thereof can be used. The boric acid and borate function to dissolve the metallic oxide and to keep the surface of the article to be treated clean, and also the boric acid and borate are not poisonous and hardly vaporize. Therefore, the method of the present invention can be carried out in the open air.

As the V-a group elements dissolved in the treating molten bath, one or more elements of vanadium (V), niobium (Nb) and tantalum (Ta) can be used, 1% by weight (hereinafter means by weight) of V-a group element in the treating molten bath being sufficient. ln practise, however, the V-a group element may be dissolved into the treating molten bath in a quantity between 2 and 20%.

In order to dissolve the V-a group elements, a block of the metal of a V-a group element or a block of the alloy containing a V-a group element can be used. As the preferred alloy, a ferrous alloy of a V-a group element is the most practical because it is relatively cheap and easily obtained. It is not preferable for said alloys to contain more than 10% of Ti, Zr, Hf, Mn, Si, Al, Mg, Ca or the rare earth elements, which metals reduce the boric acid or borate to metallic boron. In order to lower the viscosity of the treating molten bath, salt such as chloride and fluoride of alkali metal can be added into the treating molten bath.

During the anodic dissolution of the V-a group elements, the practical current density may be selected within a range from 0.2 to A/cm The increase of the current density makes the necessary time short for dissolving a certain amount of the V-a group element in the treating molten bath. The anodic dissolution of the V-a group elements can be carried out in a relatively low voltage at which the actual electrolysis of the boric acid or borate has not occurred. Although the necessary time for the anodic dissolution of the V-a group elements depends upon the current density, the volume of the treating molten bath, the size of the block as the anode and the compounds included in said block, the practical time of the anodic dissolution may be from 30 minutes to 5 hours.

The iron, ferrous alloy or cemented carbide to be treated must contain at least 0.05% of carbon and preferably contain 0.1% of carbon or higher. The carbon in the article becomes a composition of the carbide during the treatment. Namely, it is supposed that the carbon in the article diffuses to the surface thereof and reacts with the V-a group element from the treating molten bath to form the carbide on the surface of the article. The higher content of the carbon in the article is more preferable for forming the carbide layer. The iron, ferrous alloy or cemented carbide article containing less than 0.05% of carbon may not be formed with a uniform and thick carbide layer by the treatment. Also, the article containing at least 0.05% of carbon only in the surface portion thereof, can be treated to form a carbide layer on the surface of the article. For example, a pure iron article, which is case-hardened to increase the carbon content in the surface portion thereof, can be used as the article of the present invention.

Here, iron means iron containing carbon and casehardened iron, ferrous alloy means carbon steel and alloy steel, and cemented carbide means a sintered tungsten carbide containing cobalt. Said cemented carbide may include a small amount of titanium carbide, niobium carbide, tantalum carbide and the like.

In some cases, the carbon contained in the treating molten bath can be used as the source of the carbon for forming the carbide layer on the surface of the article. However, the formation of the carbide layer is not stable and the use of the carbon is the treating molten bath is not practical.

Before the treatment, it is important to purify the surface of the article for forming a good carbide layer by washing out the rust and oil from the surface of the article an acidic aqueous solution or another liquid.

The treating temperature may be selected within the wide range from the melting point of boric acid or borate to the melting point of the article to be treated. Preferably, the treating temperature may be selected within the range from 800 to l,l00C. With lowering of the treating temperature, the viscosity of the treating molten bath gradually increases and the thickness of the carbide layer formed decreases. However, at a relatively high treating temperature, the treating molten bath deteriorates rapidly. Also, the quality of the material forming the article deteriorates by increasing the crystal grain sizes of said material.

The treating time depends upon the thickness of the carbide layer to be formed. Heating shorter than 10 minutes will, however, provide no practically accepted formation of said layer, although the final determination of the treating time depends on the treating temperature. With the increase of the treating time, the thickness of the carbide layer will be increased correspondingly. In practice, an acceptable thickness of the layer can be realized within 30 hours or shorter time. The preferable range of the treating time will be from 1 to 30 hours. I

The vessel for keeping the treating molten bath of the present invention can be made of graphite or heat resistant steel.

It is not necessary to carry out the method of the present invention in the atmosphere of non-oxidation gas, but the method can be carried out into effect either under the air atmosphere or the inert gas atmosphere.

EXAMPLE 1 1,000 grams of borax was introduced into a graphite crucible and heated up to 900C for melting the borax in an electric furnace under the air, and then a metallic plate, 6 X 40 X 60mm, made of ferro-vanadiurn (containing 53.7% of vanadium) was dipped in the middle of the molten borax. With use of the metallic plate and the crucible as an anode and cathode respectively, said metallic plate was anodically dissolved into the molten borax by applying a direct-current for 2 hours at an electric currentdensity of 2A/cm of the anode. Thus a treating molten bath containing 9.8% of said ferrovanadium was prepared. I

Next, a polished specimen of 7mm diameter and made of carbon tool steel (JIS SK4, containing 1.0% of carbon) was immersed into the treating molten bath and kept therein for 2 hours, taken out therefrom and air cooled. The treating material adhered to the surface of the specimen and was removed by washing with hot water and the specimen treated was then investigated. The surface of the specimen was very smooth. After cutting and polishing the specimen, the specimen was micrographically, observed, and it was found that a uniform layer shown in FIG. 1 was formed. The thickness of the layer was about 7 microns. And the layer was identified to be vanadium carbide (VC) by the X-ray diffraction method and by an X-ray microanalyzer.

EXAMPLE 2 In the same manner as described in Example 1, a metallic plate, 6 X 40 X 50mm, made of ferro-niobium (containing 58.9% of niobium and 3.6% of tantalum) was anodically dissolved, thus a treating molten bath was prepared.

Next, a polished specimen made of carbon tool steel (JlS SK4) was immersed into the treating molten bath and kept therein for 2 hours. By the treatment, a uniform layer of 9 micron thick, which is similar to that formed in Example 1, was formed (shown in FIG. 2). The layer was identified to be niobium carbide (NbC) containing a small amount of tantalum by X-ray diffraction method and by the an X-ray microanalyzer.

EXAMPLE 3 500 grams of borax was introduced into a graphite crucible and heated up to 1000C by melting the borax in an electric furnace, and then an electrolytic niobium plate, 40 X 35 X 4mm, was anodically dissolved into the molten borax by applying a direct current for 2 hours at an electric current density of lA/cm of the surface of the anode. By calculating the loss of the weight of the plate, a treating molten bath containing about 9.4% of niobium dissolved was prepared.

Next, a specimen, 1mm thick, 5.5mm wide and 30mm long, made of cemented carbide composed of 91% of tungsten carbide and 9% of cobalt was immersed into the treating molten bath and kept therein for 14 hours, taken out therefrom and air-cooled. The treating material adhered to the surface of the specimen and was removed by dipping the specimen into hot water. The surface of the specimen treated was smooth. After cutting and polishing the specimen, the cross section of the specimen was micrographically observed and tested by the X-ray diffraction method and by an X-ray microanalyzer. Through observation, a layer having a uniform and dense structure and a 26 micron thick was found. By the X-ray diffraction method, strong niobium carbide (VC) diffraction lines were detected from the layer. In FIG. 3, the chart of the X-ray diffraction method is shown. By an X-ray microanalyzer, the layer was found to contain a large amount of niobium. The hardness of the layer measured from the surface of the specimen was about l-Iv (Micro Vickers Hardness) 2888. Also the hardness of the mother material of the specimen was measured to be about H'v 1525.

EXAMPLE 4 In the same manner as described in Example 3, an electrolytic tantalum plate, 50 X 40 X 4mm, was annodically dissolved for 1 hour at l,000C at an electric current density of 1A/cm Thus, a treating molten bath containing about 1 1.2% of tantalum dissolved was prepared.

Next, a specimen having the same size and made of the same material as the specimen used in Example 3 was immersed into the treating molten bath and kept therein for 16 hours. By the treatment, a uniform and dense layer of 15 micron thick was formed. The layer was identified as be tantalum carbide (TaC) by the X-ray diffraction method. The hardness of the layer was about HV 1720.

What is claimed is:

1. In a method for forming a carbide layer on an iron, ferrous alloy, or cemented carbide article, said articles containing at least 0.05% by weight of carbon, which comprises preparing a molten treating bath consisting essentially of boric acid or a borate and a metallic element dissolved therein, said metallic element being selected from the group consisting of vanadium, niobium and tantalum; immersing said article into said molten treating bath; maintaining said article in said molten treating bath to form a carbide layer of said metallic element on the surface of said article, and

removing said article from the molten treating bath; the

improvement wherein said molten treating bath is prepared by the method which comprises heating a boric acid or a borate to its molten state to form a molten bath; dipping a metal block containing at least one element selected from the group consisting of vanadium, niobium and tantalum, and applying an electric current to the molten bath through said metal block as the anode and an electric conductive material, which is in contact with the molten bath as the cathode, to anodically dissolve said metal block.

2. A method according to claim 1, wherein said borate is selected from the group consisting of sodium borate and potassium borate.

3. A method according to claim 1, wherein said metal block is a metallic alloy.

4. A method according to claim 3, wherein said metallic alloy is a ferrous alloy.

5. A method according to claim 1, wherein said ferrous alloy article is made of one selected from the group consisting of carbon steel and alloy steel containing at least 0.05% of carbon.

6. A method according to claim 1, wherein said cemented carbide article is made of a sintered tungsten carbide containing cobalt.

7. A method according to claim 1, wherein said treating molten bath contains at least 1% by weight of the V-a group element dissolved.

8. A method according to claim 1, wherein the electric current density of the anode is selected in a range from 0.2 to 5 A/cm and said metal block is anodically dissolved for a time ranging from 30 minutes to 5 hours.

9. A method according to claim 8, wherein the treating bath contains at least 1% by weight of theV-a group element dissolved in the molten treating bath and wherein said article is maintained in the molten treating 8 12. A method according to claim 1, wherein said article is maintained in the treating molten bath for 1 to 30 hours at a temperature ranging from 800 to l,l00C.

13. A method according to claim 1, wherein said treating molten bath contains chloride or fluoride of alkali metal for lowering the viscosity of the treating molten bath. 

1. IN A METHOD FOR FORMING A CARBIDE LAYER ON AN IRON, FERROUS ALLOY, OR CEMENTED CARRBICE ARTICLE, SAID ARTICLES CONTAINING AT LEAST 0.05% BY WEIGHT OF CARBON, WHICH COMPRISES PREPARING A MOLTEN TREATING BATH CONSISTING ESSENTIALLY OF BORIC ACID OR A BORATE AND A METALLIC ELEMENT DISSOLVED THEREIN, SAID METALLIC ELEMENT BEING SELECTED FROM THE GROUP CONSISTNG OF VANADIUM, NIOBIUM AND TANTALUM; IMMERSING SAID ARTICLE INTO SAID MOLTEN TREATING BATH; MAINTAINING SAID ARTICLE IN SAID MOLTEN TREATING BATH TO FORM A CARBIDE LAYER OF SAID METALLIC ELEMENT ON THE SURFACE OF SAID ARTICLE, AND REMOVING SAID ARTICLE FROM THE MOLTEN TREATING BATH; THE IMPROVEMENT WHEREIN SAID MOLTEN TREATING BATH IS PREPARED BY THE METHOD WHICH COMPRISES HEATING A BORIC ACID OR A BORATE TO ITS MOLTEN STATE TO FORM A MOLTEN BATH; DIPPING A METAL BLOCK CONTAINING AT LEAST ONE ELEMENT SELECTED FROM THE GROUP CONSISTING OF VANADIUM, NIOBIUM AND TANTALUM, AND APPLYING AN ELECTRIC CURRENT TO THE MOLTEN BATH THROUGH SAID METAL BLOCK AS THE ANODE AND AN ELECTRIC CONDUCTIVE MATERIAL, WHICH IS IN CONTACT WITH THE MOLTEN BATH AS THE CATHODE, TO ANODICALLY DISSOLVE SAID METAL BLOCK.
 2. A method according to claim 1, wherein said borate is selected from the group consisting of sodium borate and potassium borate.
 3. A method according to claim 1, wherein said metal block is a metallic alloy.
 4. A method according to claim 3, wherein said metallic alloy is a ferrous alloy.
 5. A method according to claim 1, wherein said ferrous alloy article is made of one selected from the group consisting of carbon steel and alloy steel containing at least 0.05% of carbon.
 6. A method according to claim 1, wherein said cemented carbide article is made of a sintered tungsten carbide containing cobalt.
 7. A method according to claim 1, wherein said treating molten bath contains at least 1% by weight of the V-a group element dissolved.
 8. A method according to claim 1, wherein the electric current density of the anode is selected in a range from 0.2 to 5 A/cm2 and said metal block is anodically dissolved for a time ranging from 30 minutes to 5 hours.
 9. A method according to claim 8, wherein the treating bath contains at least 1% by weight of the V-a group element dissolved in the molten treating bath and wherein said article is maintained in the molten treating bath for 1 to 30 hours and at a temperature ranging from 800* to 1100*C.
 10. A method according to claim 1, wherein a vessel containing the molten bath is used as the cathode during the step of applying the electric current.
 11. A method according to claim 1, wherein during the step of maintaining the article in the molten treating bath, said metal block is again anodically dissolved for supplying the V-a group elements into the molten treating bath.
 12. A method according to claim 1, wherein said article is maintained in the treating molten bath for 1 to 30 hours at a temperature ranging from 800 to 1,100*C.
 13. A method according to claim 1, wherein said treating molten bath contains chloride or fluoride of alkali metal for lowering the viscosity of the treating molten bath. 